It is generally desirable for projection systems to produce high quality images while being compact and inexpensive. In prior art transmissive systems using polysilicon liquid crystal panels, high contrast is achieved by situating each panel between crossed sheet polarizers. Color management beamsplitters/combiners and other optics are external to these units, insuring that polarization is not contaminated and contrast degraded.
In projection systems using reflective panels, such as liquid crystal on silicon (LCOS) displays, beam routing is frequently accomplished using polarizing beamsplitters (PBSs). PBSs are four-port devices, comprising an input port, an output port, and a reflective panel port, with the fourth port typically unused. The behavior of thin-film PBS cubes, for instance, is such that s-polarized light is reflected, while orthogonally polarized light is substantially transmitted.
Efficient use of ultra-high pressure mercury (UHP) lamp emission to achieve high-brightness generally requires that the projection system operate at f2.8 or lower. F-number reduction to achieve high brightness is often accompanied by diminished contrast ratio. In practice, precise separation of input and modulated light becomes challenging when PBSs are used in even modest f-number systems. The control of skew rays in such systems has previously been analyzed in the context of contrast ratio preservation. A simple quarter-wave retarder between the panel and PBS is a well-known compensation scheme for managing contrast loss due to the PBS geometrical rotation.
Compact LCOS-based projection systems can be realized by combining the beam separation function of the PBS with a polarization based color management. Retarder stacks, as described in U.S. Pat. No. 5,751,384, enable encoding of color by polarization, thereby expanding the role of neutral PBSs to function as color splitters and combiners. However, such color management systems have the same skew-ray susceptibility found in the panel ports. In order to maintain high color quality and contrast in low f-number systems, the polarization encoding incorporates skew ray correction.